My 12v battery is powering my motors but my 6v battery is powering my microcontroller and my sensors.IF overloads can't happen( i heard that when powering your robot this and short circuit is something you have to look out for)then what circuit do you suggest for protecting my batteries form shorts (what amp fuses , circuit etc...)

You can put a diode at the input of your battery connection, before any other electronics, to ensure the current only flows in one direction. Just in case you connect the battery backwards, your circuit will not get damaged. You should also use a fuse at the output of your voltage regulator so that if a short circuit does happen, the fuse will blow before the over current destroys your circuit.

Keep in mind the diodes and the fuses will have to be rated differently for your microcontroller / sensor circuit and your motor circuit.

overload? as far as I know batteries cant over load, they just wont put out the required power.

Then here's the lesson of today Batteries can overload, just try a crowbar on a car battery if you don't believe me. (Better not, as you would find yourself in the epicenter of a burst of acid and molten lead).The crowbar would be called a short circuit, but consider something between the short (eg. 0.001 Ohm) and a suitable load, eg. a (static or average) load of 1 Ohm on a 5 cell (6V) NiXx battery - that would constitute an overload in my book (or to really get the message across, change the load to 0.2 Ohm).

An overload is any load that draws a higher current than the battery used are designed for.

« Last Edit: April 23, 2010, 09:04:11 PM by Soeren »

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Regards,Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?Please remember...Engineering is based on numbers - not adjectives

I have two battery's powering my robot :[...]how can i protect my battery's from having short circuit and/or overloadwhat do you suggest i use (what kind of fuses, circuit etc...)?

A fuse for each battery placed in the positive (red) lead between the connector and the battery (be carefull when you mount them. Use inline fuse holders.As is very typically for this kind of shops, no datasheet to be found and you have no idea what they stuffed under the wrap, so to play safe, use fuses equal to their C-rating (2.0A and 2.6A) or closest equivalent - use slo-blo fuses, as you don't want the fuse to blow on short surges.

You should still use fuses of appropriate value in other places, like immediately before the voltage regulator etc. but there should allways be a fuse as physically near the positive pole of the battery as possible, as this will protect both you and the battery, should you accidentally short some point after that.

Oh and use different connectors for the two batteries, to make sure you don't get them swapped around.

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Regards,Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?Please remember...Engineering is based on numbers - not adjectives

Speaking of voltage regulators, i like to put a zener diode right after them between + and ground, a little bigger than the output of the regulator (5.6 zener for a 5 volt regulator). If the regulator fails for any reason, the zener would create a short, blowing the fuse, hopefully before anything gets damaged. But, i guess we all heard the saying that if you put a fast fuse to protect a transistor, the transistor will blow to protect the fuse.

Well, my idea was to protect what's downstream form the regulator in case it fails, not the regulator itself. If the zener is before the regulator, and it shorts sending, let's say, 15V in 5V circuits... That would cost more than a 7805, i'm sure.

Well, my idea was to protect what's downstream form the regulator in case it fails, not the regulator itself. If the zener is before the regulator, and it shorts sending, let's say, 15V in 5V circuits... That would cost more than a 7805, i'm sure.

If the zener is before the regulator and it shorts (the common failure mode for any diode btw), it will "send downstream" a safe 0V for a ms or so, until it has blown the fuse, after which it will be reduced by approximately 20.5 nV (only on work days, it's less than 10nV during weekends).That will cost you a fuse and no matter where you placed it, whatever it cost to repair and/or redesign the apparently bad design that failed in the first place

Regulators don't just break down - with a proper designed circuit they last for decades.

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Regards,Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?Please remember...Engineering is based on numbers - not adjectives

But OK, I'll type it real slow this time: 2A Slo-Blo and 2.6A Slo-Blo.This is the fuses for the batteries and should be mounted as near as possible to the positive battery terminal in the red lead. This is for protecting the batteries from shorts and overloads, as you asked about.

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Regards,Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?Please remember...Engineering is based on numbers - not adjectives

Thanks again for reply So just confirming a 5-6A fuse close as possible to the positive connector of my battery? Do i need any other fuse from the h-bridge or something(I'm using this h-bridge:http://www.st.com/stonline/books/pdf/docs/1773.pdfandif this battery wouldn't last long then what battery do you suggest : something with more mAh , NIMH battery? and i also have this battery:http://www.all-battery.com/6v200mahnimhsidebysiderxreceiverbatterypackswhitecconnectorforrcaircraftswalkingrobot-2.aspxregulated by a 5v regulator powering my microconntroller, what kind of fuse do you suggest for it ?(PS: sorry i ask so many questions I'm just quite new to this )thanks again

It won't hurt with a fuse for each motor - around 2A to 3A Slo-Blo each.It should be selected so that even the worst in-rush (start-up) surge won't blow it, but in case of a stalled motor, it should blow to protect the motor windings.I didn't check the stall current of your motors (we have to leave a little of your foot work to be done by you ), but both types of motors were a bit under 1A (0.8A and 0.9A respectively). Around 2.5 to 3 times that would be a good starting point - and allways use Slo-Blo fuses when they need to withstand surges.If you want to fuse the electronics and there's not a huge capacitor in-circuit, you would use the "F" (Fast blowing) types of fuses.

if this battery wouldn't last long then what battery do you suggest : something with more mAh , NIMH battery? and i also have this battery:

There's this saying:Give a man a fish and he may be poisoned with mercury, but teach him a bit of electronics and he never have to eat another fish. (That's how I remember it anyway )

Newbie or not, I'm sure you have learned simple addition and subtraction.Add the currents of each motor (that will be the major consumers).0.91 + 0.91 + 0.91 + 0.8 = 3.53 [A]Add about 0.5A for the remaining circuit and you're up to 4A

Battery capacity is given in Ah (Ampere hours). It's the 20 hour rate for (most) lead acid and the 5 hour rate for nearly anything else.As a rule of thumb, you can say that a 1Ah battery can supply 1A for 1 hour (or 2A for half an hour etc.). It's not entirely correct, since a 1Ah NiMH cell will be able to give 0.2A for 5 hours, but as the current increases, the capacity falls, so at 1A, you may get only 45..50 minutes (or less).

You need a battery that can provide 4A and since you probably don't wanna charge for a couple of hours every 20 minutes of use or so, you should get at least 4Ah to 5Ah cells.

Something that you could just as well learn right from the beginning:A = ampere the measure of current, allways capital Ah = hour, allways lower case hIf there's just an A, we're talking current (in ampere), if it's Ah, we're talking capacity (in ampere-hours) - never get those mixed up.

Quite OK, but don't forget Google and your own brain - being spoon fed won't teach you very much, compared to the Ahaas/Eurekas releasing endorphins to your grey matter when you solve something by yourself.

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Regards,Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?Please remember...Engineering is based on numbers - not adjectives

Quite OK, but don't forget Google and your own brain - being spoon fed won't teach you very much, compared to the Ahaas/Eurekas releasing endorphins to your grey matter when you solve something by yourself.

quite true, as asking immediately may seem like the best choice at the moment but the answer might actually be pretty obvious. I find it kind of embarrassing answering the question to my own topic. quick answers from us are what we are here for, but getting the answer from us gives you little satisfaction unlike solving the problem yourself and feeling good about figuring it out without help. Sorry if I'm getting to into this .

Morale: think before you ask, but after a while, we'd rather have you ask than kill yourself over it